Segmental arterial mediolyis (SAM), also known as segmental medial arteriolysis (SMA), is a rare vasculopathy characterized by non-inflammatory degeneration of the medial layer of muscular arteries (1
) and, occasionally, adjacent veins (12
). Originally described by Slavin and Gonzalez-Vitale as “segmental mediolytic arteritis” in 1976, Slavin and colleagues later proposed a change in name to “segmenal arterial mediolysis” due to lack of consistent evidence of true inflammation in both the clinical presentation and the histologic features of the disease (10
SAM is a pathologic diagnosis that is defined by characteristic histologic features on surgical specimens obtained from affected anatomic sites. Though it can be difficult to diagnose, clinical clues are usually present that point to the diagnosis of SAM on the basis of history, physical examination, and initial laboratory evaluation. The combination of clinical features and surgical pathology findings usually allows the discrimination of SAM from its mimics ( and ). For example, although atherosclerosis, a common vasculopathy, is usually widespread throughout many regions of the vascular tree, SAM typically is limited to vessels in only one anatomic site (3
). In addition, atherosclerosis typically occurs at the branch points of vessels in patients with traditional cardiovascular risk factors, while these features are absent in SAM (13
). Furthermore, although atherosclerosis typically occurs in middle-aged and elderly adults, SAM may present at any age.
Key vascular, histopathologic, and radiographic features of segmental arterial mediolysis (SAM) and its mimics.
It is a particular challenge to distinguish SAM from fibromuscular dysplasia (FMD), especially since SAM is often considered an early lesion of FMD (3
). Classically, FMD presents in young females and has a predisposition for the renal arteries, causing premature hypertension. SAM, on the other hand, may present at any age, has no gender predisposition, and most commonly affects the celiac artery and its branches. Arterial dissection and hemorrhage are also much more common in SAM than in classic FMD. In addition to FMD, another disease entity classified on the disease spectrum of SAM is cystic medial necrosis (CMN) (1
). However, typically, CMN occurs in the aorta and great vessels of patients with Marfan’s syndrome (14
), whereas these vessels are not typically affected by SAM.
We reviewed all cases of biopsy-confirmed SAM occurring at our institution over the past 20 years. The two cases described here represent the results of our medical record review. Although many more cases of SAM were suspected on the basis of clinical and radiologic features, these were the only two cases with histopathologic confirmation. Of note, the cases identified in our institution lacked physical signs and symptoms and laboratory indicators of systemic inflammation, helping to distinguish SAM from the inflammatory vasculitides.
The most dramatic presentation of SAM is sudden, life-threatening hemorrhage of the abdomen, retroperitoneum, or brain (3
). Hemorrhage results from either aneurysm rupture or dissections occurring as a result of weakening along the plane separating the outer media from the adventitia (3
). Interestingly, each of the cases of SAM identified at our institution presented in a relatively benign fashion, with abdominal pain as the chief complaint. Ischemic colitis, as in Case 1, has been reported as an example of a relatively less acute presentation of SAM (16
). More benign presentations of SAM could easily escape clinical diagnosis, and therefore, SAM may be substantially more common than is suggested by the literature (3
Angiography can reveal several patterns that are consistent with SAM, including single or multiple aneurysms, dissections, stenoses, and occlusions (3
). As seen in our cases and in another report by Michael and colleagues (18
), lesions of SAM may evolve rapidly over the course of weeks on serial angiography. Despite patterns on angiography that are suggestive of SAM, histopathology remains the gold standard for definitive diagnosis (). This is especially important in the case of polyarteritis nodosa (PAN), which can have an angiographic appearance identical to that of SAM (3
). Lack of inflammation on arterial biopsy in SAM allows these two entities to be readily distinguished. Patients with PAN also generally have clinical evidence of systemic inflammation, where as patients with SAM do not.
The discrimination of SAM from systemic inflammatory vasculitides is particularly important, since corticosteroids and immunosuppressive agents, which are crucial in the treatment of the inflammatory vasculitides, have no proven benefit in SAM (19
). Without any evidence of an inflammatory etiology, the use of immunosuppressive regimens in SAM exposes the patient to undue risks, including infection and poor wound healing, and could possibly worsen prognosis (19
). Treatment of SAM involves embolization, surgical bypass, or resection of the injured arteries (20
). The long-term prognosis of SAM is somewhat uncertain, since its natural history has not been thoroughly characterized despite its initial description over 30 years ago. It is known, however, that while cases of SAM complicated by intra-abdominal hemorrhage have a mortality approaching 50% (21
), the most common scenario is of long-term disease-free survival following embolization, bypass, or resection of the affected areas. There have even been reports of complete spontaneous resolution of the vascular lesions of SAM (18
). Our cases were both followed for 2 years with no recurrence of disease.
In summary, SAM is a rare but important cause of unexplained vascular lesions in patients in whom other inflammatory, infectious, or heritable diseases have been ruled out. The diagnosis should be considered when a patient presents with unexplained acute-onset abdominal pain with or without intra-abdominal bleeding. SAM should also be kept in mind when aneuysms, stenoses, and occlusions are identified in medium and large vessels, especially when these lesions are limited to one anatomic location. Conventional angiography is more sensitive than CT or MR angiography and should be used after more conventional methods of imaging are unrevealing. If a diagnosis of SAM is suspected, a multi-disciplinary approach involving consultation with interventional radiology and vascular or general surgery should be promptly pursued.